This work provides a statistical analysis of the massive star binary characteristics in the Cygnus OB2 Association using radial velocity information of 114 B3–O3 primary stars and orbital properties for the 24 known binaries .
We compare these data to a series of Monte Carlo simulations to infer the intrinsic binary fraction and distributions of mass ratios , periods , and eccentricities .
We model the distribution of mass ratio , log-period , and eccentricity as power-laws and find best fitting indices of \alpha = 0.1 \pm 0.5 , \beta = 0.2 \pm 0.4 , and \gamma = -0.6 \pm 0.3 respectively .
These distributions indicate a preference for massive companions , short periods , and low eccentricities .
Our analysis indicates that the binary fraction of the cluster is 44 \pm 8 % if all binary systems are ( artificially ) assumed to have P < 1000 days ; if the power-law period distribution is extrapolated to 10 ^ { 4 } years , a plausible upper limit for bound systems , the binary fraction is \sim 90 \pm 10 % .
Of these binary ( or higher order ) systems , \sim 45 % will have companions close enough to interact during pre- or post-main-sequence evolution ( semi-major axis \lesssim 4.7 AU ) .
The period distribution for P < 27 d is not well reproduced by any single power-law owing to an excess of systems with periods around 3–5 days ( 0.08–0.31 AU ) and a relative shortage of systems with periods around 7–14 days ( 0.14–0.62 AU ) .
We explore the idea that these longer-period systems evolved to produce the observed excess of short-period systems .
The best fitting binary parameters imply that secondaries generate , on average , \sim 16 % of the V-band light in young massive populations .
This means that photometrically based distance measurements for young massive clusters & associations will be systematically low by \sim 8 % ( 0.16 mag in the distance modulus ) if the luminous contributions of unresolved secondaries are not taken into account .